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Angiosarcoma of the thyroid has long been a controversial entity, and it is histologically defined as cleft-like anastosmosing spaces lined by large, atypical cells of endothelial lineage. However, clear-cut separation between the angiosarcoma and anaplastic carcinoma of the thyroid is difficult because they yield nearly the same clinical prognosis and overlapping histologic findings. We report a case of thyroid neoplasm composed of minimally invasive well differentiated follicular carcinoma and angiosarcoma with intervening transitional area. Immunohistochemically, the angiosarcomatous portion showed focal immunoreactivity for endothelial markers such as CD31, CD34, Ulex europaeus 1 lectin, factor VIII-related antigen, and immunonegativity for epithelial markers including pancytokeratin, epithelial membrane antigen and thyroglobulin, whereas the reverse was demonstrated in the minimally invasive follicular carcinomatous portion. The follicular carcinoma portion was positive for thyroid transcription factor-1 (TTF-1). Each component showed ultrastructural findings of epithelial and endothelial differentiation, respectively. The present case was unique in that angiosarcoma of the thyroid was confirmed by immunohistochemistry and electron microscopy, as well as light microscopy, and also coexisted with a minimally invasive well differentiated follicular carcinoma in the same mass. This combination has never been described in the literature. Although restricted to a single case, the present case further supports that angiosarcoma is a true existent entity rather than a variant of anaplastic carcinoma.